In estimation of a state-of-charge (hereinafter, referred to as SOC) of a battery, for a case where no charging and discharging are performed, a mode in which an SOC is estimated according to an open circuit voltage (hereinafter, referred to as OCV) is known, and, for a case where charging or discharging is performed, a mode in which an SOC is estimated according to electric current integration and a mode in which an SOC is estimated by using an equivalent circuit model, are known.
That is, as a mode in which an SOC is constantly estimated regardless of charging or discharging of a battery, two modes are known, i.e., a mode (hereinafter, referred to as electric current integration mode) in which an SOC is updated through electric current integration by using, as an initial value, an SOC estimated according to an OCV, and a mode (hereinafter, referred to as equivalent circuit model mode) in which an SOC is updated with the use of an equivalent circuit model by similarly using, as an initial value, an SOC estimated according to an OCV, are known.
In the former electric current integration mode, an electric current detection error of an electric current sensor exerts an influence on estimation accuracy of an SOC, and estimation accuracy of an SOC is lowered with the elapse of time. Meanwhile, in the latter equivalent circuit model mode, estimation accuracy of a circuit parameter exerts an influence on estimation accuracy of an SOC, and estimation accuracy of a circuit parameter is low when an operation of a battery is started, and the estimation accuracy of an SOC is enhanced with the elapse of time.
In order to address this, a method in which switching between an SOC estimate value according to the electric current integration mode and an SOC estimate value according to the equivalent circuit model mode is performed at a predetermined timing, is suggested (see Patent Document 1).
In this conventional art, an SOC estimate value according to the electric current integration mode is switched to an SOC estimate value according to the equivalent circuit model mode, under a condition that variation in electric current and voltage is small, to calculate a final SOC.